The preganglionic fibers are myelinated, while the postganglionic fibers are unmyelinated. The unmyelinated fibers are sur-rounded by the cytoplasm of Schwann cells, and a single Schwann cell envelopes several axons.
The viscerosensory fibers are myelinated. They are regarded as being neither sympa-thetic nor parasympathetic in nature. In general, they accompany the sympathetic nerves and enter the spinal cord through the posterior roots. The fibers from the heart run through the upper thoracic roots, those from the stomach, liver, and gallblad-der run through the middle thoracic roots, and those from the colon and appendix run through the lower thoracic roots. The re-spective dermatomes of these roots corre-spond roughly to the various Head’s zones.